Advanced thermal protection materials will have to meet demands for an acceptable environment. Smoke toxicity, outgassing, dust and other irritants are a problem not only for humans but also for equipment.
Current thermal protection materials in aircraft passenger cabins are a major problem because most common thermoplastic materials are unacceptable because they are flammable, and can generate toxic fumes. For application in spacecraft, satellites, and military aircraft, smoke generation or outgassing may contaminate optical surfaces or react chemically with machine components. These pollutants can be controlled in part by the selection of fibers, coatings, and proper pre- or post-treatments to minimize outgassing. Most applications for advanced aircraft require quantitative limits for volatile materials. Highly crystalline, fully cross linked or thermosetting polymeric materials have been used where relatively inert behavior is required. However, such materials are still flammable.
The prior art has used asbestos, glass wool, polyester and polypropylene fibers, carbon and graphite short straight staple felts, fowl down an various foam materials such as polyurethane foam as thermal insulation for many applications. While asbestos, carbon and graphite felts and fiber glass are considered non-flammable, the other aforementioned thermal insulating materials are considered flammable. The bulk densities of some of the well known thermal insulating materials are in the range of 0.35 to 2 pounds per cubic foot (5.6 -32.04 kg/m.sup.3) for insulating materials useful at temperatures not exceeding 120 degrees C. to 2-5 plus pounds per cubic foot for the high temperature insulating materials. Even the newest "light weight" insulating material recently disclosed by NASA consisting of a ceramic from which a carbonaceous material has been burned out, has a bulk density of about 2-6 pounds per cubic foot (32-96 kg/m.sup.3). In addition many of the thermal light weight thermal insulation material which is a blend of spun and drawn, crimped, staple, synthetic polymeric microfibers having a diameter of from 3 to 12 microns, and synthetic polymeric staple microfibers having a diameter of more than 12 and up to 50 microns. However, the insulation material is not fireproof and does not provide good sound absorbing properties.
U.S. Pat. No. 4,167,604 to William E. Aldrich discloses the use of crimped hollow polyester filaments in a blend with down in the form of a multiple ply carded web which is treated with a thermosetting resin to form a bat having thermal insulating characteristics. The web, however, does not have fireproof characteristics and is not a good sound absorbent.
U.S. Pat. No. 4,321,154 to Francois Ledru relates to high temperature thermal insulation material comprising insulating mineral fibers and pyrolytic carbon. To make the insulation light weight an expanding agent is utilized or hollow particles such as microspheres are utilized.
U.S. Pat. No. 4,193,252 to Shepherd, et al. discloses the preparation of partially carbonized, graphite and carbon fibers from rayon which has been knitted into a fabric assembly. When the fabric is deknitted, the partially carbonized and the carbonized fibers contain kinks. The fully carbonized or graphite fibers have kinks which are more permanent in nature. Applicants have found that partially carbonized rayon fibers do not retain their reversible deflection and lose their kinks at relatively low temperatures or under tension. The fully carbonized or graphite yarn which is prepared from rayon is brittle and difficult to handle when deknitting. Moreover, carbon fibers produced from rayon are known to possess high water absorption and lower thermal conductivity than fibers with a higher graphite content, such as fibers prepared from acrylic fibers.